A. Field of the Invention
This invention relates generally to integrated optics and photonics and more particularly to waveguides for performing splitting and amplification of light signals and to methods for making such waveguides.
Applications for optically pumped waveguide splitter/amplifiers include, but are not limited to, integrated optical architectures for signal processing, VLSI intraconnects and interconnects, fiber optic communications systems, antenna remoting, cable television, waveguide sensing, and control of phased array antennae.
B. Description of Related Art
Diffraction gratings illuminated with a highly spatially coherent plane wave produce "self-images" in certain planes behind the grating. The lens-like imaging produced solely by free-space propagation of a diffracted field is known as the Talbot effect. W. H. F. Talbot, "Facts relating to optical science, No. IV," Philos. Mag. 9, 401-407 (1836). Typical applications of the Talbot effect include optical image processing, optical testing, and production of optical elements. An overview of the operation of waveguide self-imaging devices is found in L. B. Soldano and E. C. M. Pennings, "Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and Applications," J. of Lightwave Technology 13, 615-627 (April 1995).
The use of the self-imaging effect in waveguides to perform 1 to N way beamsplitting is known, as exemplified by U.S. Pat. No. 5,475,776 to Jenkins et al., "Optical Mixing Device;" U.S. Pat. No. 5,410,625 to Jenkins et al., "Optical Device for Beam Splitting and Recombining;" U.S. Pat. No. 5,428,698 to Jenkins et al., "Signal Routing Device;" and U.S. Pat. No. 5,640,474 to Tayag, "Easily Manufacturable Optical Self-Imaging Waveguide."
U.S. Pat. Nos. 5,838,842 and 5,852,691 to Mackie, issued on Nov. 17, 1998 ("'842 patent") and Dec. 22, 1998 ("'691 patent"), respectively, provide additional background on self-imaging multimode interference devices and are incorporated by reference herein as if fully set forth. The '842 and '691 patents discuss techniques for the separation of orthogonally polarized light or that of 2 arbitrary wavelengths by use of a waveguide optical device, based on 1) simultaneous crossed and barred 1-by-1 off-center self-imaging, 2) out-of-phase self-imaging, and 3) simultaneous 1-by-1 and 1-by-2 self-imaging. Simultaneous crossed and barred 1-by-1 off-center self-imaging utilizes a waveguide optical polarization splitter having an input waveguide containing TE and TM, a multimode interference device, aligned so that TE and TM refractive indices are very different, and with the length set so that the polarization with the lower refractive index is bar self-imaged while the other polarization is cross self-imaged, an output waveguide containing polarization with higher refractive index, and another output waveguide containing the other polarization. Out-of-phase self-imaging utilizes a waveguide optical polarization splitter having an input waveguide containing TE and TM, a 1-by-2 polarization-independent self-imaging power splitter, an intermediate waveguide of length L.sub.par containing TE and TM, an intermediate waveguide of length L.sub.par +2*L.sub.perp containing TE and TM, a 2-by-2 self-imaging coupler, output waveguide containing TE only, and an output waveguide containing TM only. The techniques of the '842 and '691 patents may also be used to separate light of 2 arbitrary wavelengths.
U.S. Pat. No. 5,862,288 issued on Jan. 19, 1999 to Tayag et al., incorporated by reference as if fully set forth, describes various techniques for implementing wavelength division (de)multiplexing operations using self-imaging waveguide devices. The techniques of Tayag and Batchman are similar to the techniques described in the '842 and '691 patents; however, the '842 and '691 patents discuss general wavelength splitting (e.g., the commonly used fiber-optic communication wavelength of 1.55 microns and a pump at 980 nm), not simply splitting of first and second harmonics.
There are presently two major approaches to making optical splitter/amplifiers: electrical pumping and optical pumping. Electrical pumping has the great potential advantages of simplicity and efficiency since no pump light source is needed. However, it is so extremely noisy as to render it useless for most applications. Research continues, but the problem may be intrinsic to the approach. Optical pumping, on the other hand, is well-established and in widespread commercial use in the form of erbium-doped fiber amplifiers (EDFAs). However, the EDFA is actually only one part of an optical splitter/amplifier. The pump and signal must both be introduced into the EDFA, which requires a multiplexer. Generally, a fused-fiber coupler is used. After the EDFA, the remaining pump light must be removed with a demultiplexer. Since the pump is still orders of magnitude more intense than the signal, the demultiplexer must be very efficient; a simple filter won't work. Generally, a fiber grating or bulk spectrometer is used. Lastly, the amplified signal is split, using either another fused-fiber coupler (a different type) or a waveguide splitter.
Further information and additional references on currently available splitter/amplifiers may be obtained from the patent applications and publications cited above. In summary, electrical pumping techniques are inadequate for most applications due to poor signal-to-noise ratio, and currently available optical pumping techniques use numerous components, some or all of which are bulk or fiber.